At 6:05 PM Central Time on Wednesday, August 1, 2007, the I-35W bridge
suffered structural failure and collapsed into the Mississippi River. The
accident happened at the height of rush hour with an estimated 100 cars on
the structure. There were many survivors, but tragically, a number of people
were not so fortunate. As of August 21, 2007, all known victims of the bridge
disaster have been recovered. The toll stands at 13 dead and 144 injured.
The NTSB investigated for 16 months before determining the cause of the
disaster to be a design flaw triggered by several dump-truck loads of sand
and gravel placed on the bridge as part of a repair project.

• Structure ID:

NBI: 9340.

• Location:

River Mile 853.20.

• River Elevation:

725 Feet.

• Highway:

I-35W.

• Daily Traffic Count:

140,000 (2002), 141,000 (2005).

• Bridge Type:

Three Span Continuous Deck Truss Bridge.

• Length:

1,907 Feet Overall, 458 Foot Longest Span.

• Width:

108 Feet Curb To Curb, 113 Feet 4 Inches Overall.

• Traffic Lanes:

8 Total, 6 Thru Lanes, 2 Auxiliary Lanes.

• Navigation Channel Width:

390 Feet.

• Height Above Water:

64 Feet To Low Steel, 116 Foot Deck Height.

• Date Built:

Opened November 1967, Failed August 1, 2007.

Note, this page is for the old I-35W Bridge that failed in August, 2007.
For the new I-35W Saint Anthony Falls Bridge, please
click here. For complete coverage of the
bridge disaster, please click here.

Interstate highways came to the Minneapolis area in the 1960s. The
first section of I-35W to be built was the Minnesota River crossing
from MN-13 in Burnsville and continuing north through Bloomington. This was
planned to be part of a US-65 freeway project, but was converted to
Interstate highway when that program was established. This section
was completed in 1960. The Crosstown Commons opened in 1963, and south
Minneapolis section was completed in 1967. The I-94 commons opened in
1968. The Mississippi River bridge was completed in 1967, but I-35W did
not extend across the river until 1971, and then, it ended at University
Avenue. The section from University Avenue to the canceled I-335 opened
in 1973, the section from I-335 to the present day Quarry shopping center
opened in 1975, and the remainder of I-35W to MN-36 opened in 1977.

The I-35W bridge was built at a time when technology was king and
us Americans could do no wrong. After all, we were going to the moon,
so how difficult could some bridge be to build? At the same time, we
were building an enormous number of interstate highway miles, so they
had to be built fast and inexpensively if we were going to be able to
finish the entire highway system. The I-35W bridge suffered from this
mentality. There was little oversight in the design, and few considerations
for future inspections and maintenance. More critical was the lack of
redundancy in the structure. It was built such that if a key structural
component failed, the entire bridge could fail. Much was learned about these
problems in the aftermath of the Silver Bridge disaster in Ohio, but the
I-35W bridge was built about a year too early to benefit from those
learnings. The Silver Bridge failed due to a single defect in the steel
in one connecting pin that was located in an area that was impossible to
inspect.

The bridge design is a continuous deck truss. This means that it is a
truss structure made up of a lattice of metal beams. A deck truss has
the roadway on top of the steel structure rather than under or through
the steel lattice. And a continuous truss means that the metal structure
spans more than two sets of piers. In this case, there are 4 sets of
piers with the truss crossing over three spans. The main Mississippi
River span is 458 feet long. The main span was built this way to avoid
putting bridge piers into the river, which was now part of the 9-foot
navigation channel following the opening the Saint Anthony Falls Locks
earlier in the 1960s. While the main span was 458 feet long, the
approach spans were 265 feet each, and connector spans of 36 and 40
feet were at the ends of the bridge, for a total truss length of 1,064
feet. The remainder of the bridge was built from steel girders,
adding another 843 feet for a total bridge length of 1,907 feet.

Once the bridge was finished in 1967, it was opened to local traffic.
The traffic deck was marked for 2 lanes of through traffic in each direction.
The bridge served as a detour while the fate of the nearby 10th Avenue
Bridge was debated. The Interstate highway finally opened in 1971,
though it extended only a mile north of the bridge.

The I-35W bridge was remodeled three times after it was completed. The first
was in 1977, where the deck was milled, and then new concrete was poured on
the roadway. The NTSB reports that this increased the dead weight of the
bridge by 13%. The second renovation was in 1998. The railings were
replaced, allowing 8 lanes of traffic to flow over the bridge. In addition,
an anti-ice system was installed. The final renovation was underway
in 2007 at the time of the accident. This project was to make a number
of repairs, with the major work being removal and replacement of the top
two inches of concrete on the roadway.

Due to the bridge deck being so thin, this bridge suffered from repeated
problems with black ice. MN-DOT installed a spray system to apply an ice
melt solution to the bridge surface when icing is possible. This system
consisted of 38 valves and 76 spray nozzles. The spray nozzles were
installed into holes bored into the deck. They worked much like lawn
sprinklers to shoot an antifreeze solution onto the deck across the
traffic lanes. This system proved to be so successful that it has been
installed on other bridges within the state.

The National Bridge Inventory contains a report on this bridge from 2003.
It reports the following items:

Deck Condition: Fair.

Superstructure Condition: Poor.

Substructure Condition: Satisfactory.

Scour: Foundations determined to be stable.

Bridge Railings: Meets currently acceptable standards.

Structural Evaluation: Meets minimum tolerable limits to be left in place as-is.

Water Adequacy Evaluation: Superior to present desirable criteria.

Bridge Sufficiency Rating: 50%

A University of Minnesota Civil Engineer in a report to MN-DOT recently noted
that this bridge is considered to be a non-redundant structure. That
is, if any one member fails, the entire bridge can collapse. A key
factor is that there are only four pylons holding up the arch. Any
damage to any one pylon would be catastrophic. The textbook example
of a non-redundant bridge is the Silver Bridge over the Ohio River.
It failed shortly before Christmas in 1967 resulting in 46 deaths. A
single piece of hardware failed due to a tiny manufacturing defect.
But that piece was non-redundant, and the entire bridge collapsed into
the icy river. Today, bridge engineers design bridges so that
any single piece of the bridge can fail without causing the entire
bridge to collapse. It is tragic that the I-35W bridge was built a
few years too early to benefit from that lesson.

Governor Tim Pawlenty has since ordered inspections all deck truss bridges
in the state. There are two similar bridges located in the Twin Cities area,
the
Highway 23 DeSoto Bridge in
Saint Cloud and the MN-243 Bridge over
the Saint Croix River in Osceola, Wisconsin. A third deck truss bridge is
located on Highway 123 over the Kettle River in Sandstone, MN. The
Sauk Rapids Bridge
has now been added to the inspection list. The Sauk Rapids Bridge is
unique in this list in that it is scheduled to be demolished soon due
to a new bridge being built to replace it. In addition, the
I-90 Dresbach Bridge over
the Mississippi River main channel near La Crosse, Wisconsin, is a
non-redundant style bridge that is on MN-DOT's watch list. MN-DOT has an
active study to examine alternatives that include removing the Dresbach
Bridge, which potentially has many years of remaining life, and replacing
it with a bridge that has built-in redundancy.

The photo above shows the two piers on the south end of the I-35W bridge.
Engineers are focusing on the metal joint just above the pier on the right
side as the location of the failure. Below is a close-up of the southwest
pier of the bridge. Notice the amount of rust. The triangle piece of metal
is the top of the bridge bearing, a device that is supposed to allow the
bridge to slide back and forth as the bridge expands and contracts during
heating and cooling cycles. The bearings had failed on this bridge many
years ago. This prevented the bridge from moving freely, so it had to flex
and bind instead, putting additional loads on the structure.

Above, you can see the south end of the I-35W bridge under one of the
main arches of the 10th Avenue Bridge. This
view shows just how close these two structures were. The
bottom picture is the top deck of the I-35W bridge, looking from the
parking lot of the former Hardees burger joint on the north east side
of the bridge. The Metrodome is visible in the background. The
hometown Minnesota Twins had just started a game when the
I-35W bridge collapsed.